Steel plate for producing light structures and method for producing said plate

ABSTRACT

A steel plate, the chemical composition of which includes, the contents being expressed by weight: 0.010%≤C≤0.20%, 0.06%≤Mn≤3%, Si≤1.5%, 0.005%≤Al≤1.5%, S≤0.030%, P≤0.040%, 2.5%≤Ti≤7.2%, (0.45×Ti)−0.35%≤B≤(0.45×Ti)+0.70%, and optionally one or more elements chosen from: Ni≤1%, Mo≤1%, Cr≤31, Nb≤0.1%, V≤0.1%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.

The invention relates to the manufacture of plates or structural partsmade of steel that combines both a high elastic modulus E, a low densityd and a high tensile strength.

BACKGROUND

It is known that the mechanical performance of structural elementsvaries as E^(x)/d, the coefficient x depending on the mode of externalstressing (for example in tension or in bending) or on the geometry ofthe elements (plates, bars). This illustrates the benefit of havingmaterials exhibiting both a high elastic modulus and a low density.

This requirement applies most particularly in the automotive industrywhere vehicle lightening and safety are constant preoccupations. Thusthe aim is to increase the elastic modulus and reduce the weight ofsteel parts by incorporating ceramic particles of various types, such ascarbides, nitrides, oxides or borides. The reason for this is that suchmaterials have a markedly higher elastic modulus, ranging from about 250to 550 GPa, than that of base steels, which is around 210 GPa, intowhich they are incorporated. In this way, hardening is achieved by loadtransfer between the matrix and the ceramic particles under theinfluence of a stress. This hardening is increased further due to thematrix grain size refinement by the ceramic particles. To manufacturethese materials comprising ceramic particles distributed uniformly in asteel matrix, processes are known that are based on powder metallurgy:firstly, ceramic powders of controlled geometry are produced, thesebeing blended with steel powders, thereby corresponding, for the steel,to an extrinsic addition of ceramic particles. The powder blend iscompacted in a mold and then heated to a temperature such that thisblend undergoes sintering. In a variant of the process, metal powdersare blended so as to form the ceramic particles during the sinteringphase. Despite mechanical properties improved over steels not containinga dispersion of ceramic particles, this type of process suffers fromseveral limitations:

it requires careful smelting and processing conditions in order not tocause a reaction with the atmosphere, taking into account the highspecific surface area of metal powders;

even after the compacting and sintering operations, residual poreslikely to act as initiation sites during cyclic stressing may possiblyremain;

the chemical composition of the matrix/particle interfaces, andtherefore their cohesion, is difficult to control given the surfacecontamination of the powders before sintering (presence of oxides andcarbon);

when the particles are added in large quantity, or when certain largeparticles are present, the elongation properties decrease;

this type of process is suitable for low-volume production but cannotmeet the requirements of mass production in the automotive industry; and

the manufacturing costs associated with this type of manufacturingprocess are high.

In the case of light alloys, manufacturing processes are also known thatare based on the extrinsic addition of ceramic powders into the liquidmetal. Here again, these processes suffer from most of theabovementioned drawbacks. More particularly, the difficulty ofhomogeneously dispersing the particles may be mentioned, such particleshaving a tendency to agglomerate or to settle in or float on the liquidmetal.

Among the known ceramics that could be used to increase the propertiesof steel is in particular titanium diboride TiB₂, which has thefollowing intrinsic characteristics:

Elastic modulus: 565 GPa;

Relative density: 4.52.

However, since the manufacturing processes rely on extrinsic additionsof TiB₂ particles, they suffer the abovementioned drawbacks.

SUMMARY OF THE INVENTION

The object of the invention is to solve the above problems, inparticular the availability of economically mass-manufacturing steelswith an elastic modulus increased by the presence of TiB₂ particles. Theobject of the invention is in particular to provide a continuous castingmanufacturing process that does not have particular difficulties whencasting the steels.

Another object of the invention is to provide steels having the highestpossible amount of TiB₂ particles dispersed uniformly in the matrix.

Another object of the invention is to provide high-tensile strengthsteels, the uniform elongation of which is equal to or greater than 8%,which can be readily subjected to various welding processes, especiallyresistance welding.

For this purpose, one subject of the invention is a steel plate, thechemical composition of which comprises, the contents being expressed byweight: 0.010%≤C≤0.20%; 0.06%≤Mn≤3%; Si≤1.5%; 0.005%≤Al≤1.5%; S≤0.030%;P≤0.040%, titanium and boron in amounts such that: 2.5%≤Ti≤7.2%;(0.45×Ti)−0.35%≤B≤(0.45×Ti)+0.70%, optionally one or more elementschosen from: Ni≤1%; Mo≤1%; Cr≤3%; Nb≤0.1%; V≤0.1%, the balance of thecomposition consisting of iron and inevitable impurities resulting fromthe smelting.

Preferably, the titanium and boron contents, expressed in % wt, are suchthat: −0.22≤B−(0.45×Ti)≤0.35.

Preferably, the titanium and boron contents, expressed in % wt, are suchthat: −0.35≤B−(0.45×Ti)≤0.22.

Preferably, the titanium content is such that: 4.6%≤Ti≤6.9%.

According to one particular embodiment, the titanium content is suchthat: 4.6%≤Ti≤6%.

Preferably, the carbon content is such that: C≤0.080%.

According to a preferred embodiment, the carbon content satisfies:C≤0.050%.

Preferably, the chromium content is such that: Cr≤0.08%.

The subject of the invention is also a steel plate of the abovecomposition, comprising TiB₂ and optionally Fe₂B eutectic precipitates,the mean size of which is equal to or less than 15 microns, andpreferably equal to or less than 10 microns.

Preferably, more than 80% by number of the TiB₂ precipitates are ofsingle-crystal character.

Another subject of the invention is a steel plate according to the abovefeatures, the mean grain size of said steel being equal to or less than15 microns, preferably equal to or less than 5 microns and verypreferably less than 3.5 microns.

Another subject of the invention is a steel plate as claimed in one ofthe above features, the elastic modulus of which, measured in therolling direction, is equal to or greater than 230 GPa, preferably equalto or greater than 240 GPa or preferably equal to or greater than 250GPa.

According to one particular embodiment, the tensile strength of thesteel plate is equal to or greater than 500 MPa and its uniformelongation is equal to or greater than 8%.

Another subject of the invention is an object manufactured from aplurality of steel parts, of identical or different composition and ofidentical or different thickness, at least one of said steel parts beinga steel plate according to any one of the above features, which iswelded to at least one of the other parts of this object, thecomposition or compositions of the other steel parts comprising, byweight: 0.001-0.25% C; 0.05-2% Mn; Si≤0.4%; Al≤0.1%; Ti<0.1%; Nb<0.1%;V<0.1%; Cr<3%; Mo<1%; Ni<1%; B<0.003%, the balance of the compositionconsisting of iron and inevitable impurities resulting from thesmelting.

Another subject of the invention is a process in which a steel issupplied with any one of the above compositions and said steel is castin the form of a semifinished product, the casting temperature notexceeding more than 40° C. above the liquidus temperature of said steel.

According to one particular embodiment, said semifinished product iscast in the form of a thin slab or thin strip between counter-rotatingrolls.

The cooling rate during solidification of the casting is preferableequal to or greater than 0.1° C./s.

According to one particular embodiment, said semi-finished product isreheated before it is hot-rolled, the temperature and the duration ofthe reheat both being chosen in such a way that the density of the TiB₂and optionally Fe₂B eutectic precipitates, with a maximum size L_(max)greater than 15 microns and an aspect ratio f>5, is less than 400/mm².

According to one particular embodiment, a hot-rolling operation iscarried out on the semifinished product, optionally a cold-rollingoperation and an annealing operation, the rolling and annealingconditions being adjusted in such a way that a steel plate with a meangrain size equal to or less than 15 microns, preferably equal to or lessthan 5 microns and very preferably less than 3.5 microns, is obtained.

Preferably, the hot rolling is carried out with an end-of-rollingtemperature below 820° C.

According to one particular embodiment, at least one blank is cut from asteel plate according to one of the above embodiments, or manufacturedaccording to one of the above embodiments, and the blank is deformedwithin a temperature range from 20° to 900° C.

Another subject of the invention is a manufacturing process in which atleast one steel plate according to one of the above embodiments, or asteel plate manufactured according to one of the above embodiments, iswelded.

Another subject of the invention is the use of a steel plate or of anobject according to one of the above embodiments, or a steel platemanufactured according to one of the above embodiments, for themanufacture of structural parts or reinforcing elements in theautomotive field.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent overthe course of the description below, given by way of nonlimiting exampleand with reference to the appended figures in which:

FIGS. 1 and 2 illustrate respectively the microstructure of two steelsaccording to the invention comprising an Fe—TiB₂ eutectic precipitation,in the as-cast state;

FIG. 3 illustrates the microstructure of a steel according to theinvention in the cold-rolled and annealed state;

FIGS. 4 and 5 illustrate the microstructure of two steels according tothe invention, comprising Fe—TiB₂ and Fe—Fe₂B eutectic precipitations,in the as-cast state and hot-rolled state respectively; and

FIGS. 6 and 7 illustrate the microstructure of a steel according to theinvention, cooled at two cooling rates during solidification, in theas-cast state.

DETAILED DESCRIPTION

As regards the chemical composition of the steel, the carbon content isadapted for the purpose of economically achieving a given level of yieldstrength or tensile strength. The carbon content also enables the natureof the matrix microstructure of the steels according to the invention tobe controlled, which microstructure may be partially or completelyferritic, bainitic, austenitic or martensitic, or may comprise a mixtureof these constituents in proportions suitable for meeting the requiredmechanical properties. A carbon content equal to or greater than 0.010%enables these various constituents to be obtained.

The carbon content is limited because of the weldability: the cold crackresistance and the toughness in the HAZ (Heat Affected Zone) decreasewhen the carbon content is greater than 0.20%. When the carbon contentis equal to or less than 0.050% by weight, the resistance weldability isparticularly improved.

Because of the titanium content of the steel, the carbon content ispreferably limited so as to avoid primary precipitation of TiC and/orTi(C,N) in the liquid metal. These precipitates, which form in theliquid, are deleterious to castability in the process for continuouslycasting the liquid steel. However, when this precipitation occurs in thesolidification range or in solid phase, it has a favorable effect on thestructural hardening. The maximum carbon content must therefore bepreferably limited to 0.080% so as to produce the TiC and/or Ti(C,N)precipitates predominantly during eutectic solidification or in thesolid phase.

In an amount equal to or greater than 0.06%, manganese increases thehardenability and contributes to the solid-solution hardening andtherefore increases the tensile strength. It combines with any sulfurpresent, thus reducing the risk of hot cracking. However, above amanganese content of 3% by weight, there is a greater risk of forming adeleterious banded structure arising from any segregation of themanganese during solidification.

Silicon contributes effectively to increasing the tensile strengththanks to solid solution hardening. However, excessive addition ofsilicon causes the formation of adherent oxides that are difficult toremove during a pickling operation, and the possible appearance ofsurface defects due in particular to a lack of wettability in hot-dipgalvanizing operations. To maintain good coatability properties, thesilicon content must not exceed 1.5% by weight.

In an amount equal to or greater than 0.005%, aluminum is a veryeffective element for deoxidizing the steel. However, above a content of1.5% by weight, excessive primary precipitation of alumina takes place,causing castability problems.

In an amount greater than 0.030%, sulfur tends to precipitate inexcessively large amounts in the form of manganese sulfides which verygreatly reduce the capability of undergoing hot forming or cold forming.

Phosphorus is an element known to segregate at the grain boundaries. Itscontent must not exceed 0.040% so as to maintain sufficient hotductility, thereby avoiding cracking, and to prevent hot cracking duringwelding.

Optionally, nickel or molybdenum may be added, these elements increasingthe tensile strength of the steel. For economic reasons, these additionsare limited to 1% by weight.

Optionally, chromium may be added to increase the tensile strength. Italso allows larger quantities of borides to be precipitated. However,its content is limited to 3% by weight so as to manufacture a lessexpensive steel.

A chromium content equal to or less than 0.080% will preferably bechosen. This is because an excessive addition of chromium results inmore borides being precipitated, but these are then (Fe, Cr) borides.

Also optionally, niobium and vanadium may be added in an amount equal toor less than 0.1% so as to obtain complementary hardening in the form offine precipitated carbonitrides.

Titanium and boron play an important role in the invention.

In a first embodiment, the weight contents expressed in percent oftitanium and boron of the steel are such that:2.5%≤Ti≤7.2%; and(0.45×Ti)−0.35%≤B≤(0.45×Ti)+0.70%.

The second relationship can be expressed equivalently as:−0.35≤B−(0.45×Ti)≤0.70.

The reasons for these limitations are the following:

-   -   when the weight content of titanium is less than 2.5%, TiB₂        precipitation does not occur in sufficient quantity. This is        because the volume fraction of precipitated TiB₂ is less than        5%, thereby precluding a significant change in the elastic        modulus, which remains less than 220 GPa;    -   when the weight content of titanium is greater than 7.2%, coarse        primary TiB₂ precipitation occurs in the liquid metal and causes        castability problems in the semifinished products;    -   if the weight contents of titanium and boron are such that:    -   B−(0.45×Ti)>0.70, there is excessive Fe₂B precipitation, which        degrades the ductility; and    -   if the titanium and boron weight contents are such that:    -   B−(0.45×Ti)<−0.35, the amount of titanium dissolved at room        temperature in the matrix is greater than 0.8%. No Significant        beneficial technical effect is then obtained, despite the higher        cost of adding titanium.

According to a second embodiment of the invention, the titanium andboron contents are such that: −0.22≤B−(0.45×Ti)≤0.35:

-   -   when B−(0.45×Ti)≤0.35, Fe₂B precipitation is greatly reduced,        thereby increasing the ductility; and    -   when B−(0.45×Ti).gtoreq.−0.22, the amount of titanium dissolved        in the matrix is very low, which means that the additions of        titanium are particularly effective from an economic standpoint.

According to one particular embodiment of the invention, the titaniumand boron contents are such that: −0.35≤B−(0.45×Ti)<−0.22:

-   -   when the quantity B−(0.45×Ti) is equal to or greater than −0.35        and less than −0.22, the amount of titanium dissolved at ambient        temperature in the matrix is between 0.5% and 0.8% respectively.        This amount proves to be particularly suitable for obtaining        precipitation composed solely of TiB₂.

According to one particular embodiment of the invention, the titaniumcontent is such that: 4.6%≤Ti≤6.9%. The reasons for these limitationsare the following:

-   -   when the weight content of titanium is equal to or greater than        4.6%, TiB₂ precipitation takes place in such a way that the        precipitated volume fraction is equal to or greater than 10%.        The elastic modulus is then equal to or greater than about 240        GPa; and    -   when the weight content of titanium is equal to or less than        6.9%, the amount of TiB₂ primary precipitates is less than 3% by        volume. The total TiB₂ precipitation, consisting of possible        primary precipitates and eutectic precipitates, is then less        than 15% by volume.

According to another preferred embodiment of the invention, the titaniumcontent is such that: 4.6%≤Ti≤6%. When the weight content of titanium isequal to or less than 6%, the castability is then particularlysatisfactory because of the slight precipitation of primary TiB₂ in theliquid metal.

According to the invention, Fe—TiB₂ eutectic precipitation occurs uponsolidification. The eutectic nature of the precipitation gives themicrostructure formed a particular fineness and homogeneity advantageousfor the mechanical properties. When the amount of TiB₂ eutecticprecipitates is greater than 5% by volume, the elastic modulus of thesteel measured in the rolling direction can exceed about 220 GPa. Above10% by volume of TiB₂ precipitates, the modulus may exceed about 240GPa, thereby enabling appreciably lightened structures to be designed.This amount may be increased to 15% by volume in order to exceed about250 GPa, in particular in the case of steels comprising alloyingelements such as chromium or molybdenum. This is because when theseelements are present, the maximum amount of TiB₂ that can be obtained inthe case of eutectic precipitation is increased.

The boron and titanium contents according to the invention preventcoarse primary precipitation of TiB₂ in the liquid metal. The formationof these primary precipitates of occasionally large size (measuringseveral tens of microns) must be avoided because of their deleteriousrole with respect to damage or fracture mechanisms during subsequentmechanical stressing. Moreover, these precipitates present in the liquidmetal, when they do not settle, are locally distributed and reduce theuniformity of the mechanical properties. This premature precipitationmust be avoided as it may lead to nozzle blockage when continuouslycasting the steel as a result of precipitate agglomeration.

As explained above, titanium must be present in a sufficient amount tocause endogenous TiB₂ formation in the form of Fe—TiB₂ eutecticprecipitation. According to the invention, titanium may also be presentby being dissolved at ambient temperature in the matrix in asuperstoichiometric proportion relative to boron, calculated on thebasis of TiB₂.

When the content of titanium in solid solution is less than 0.5%, theprecipitation takes place in the form of two successive eutectics:firstly Fe—TiB₂ and then Fe—Fe₂B, this second endogenous precipitationof Fe₂B taking place in a greater or lesser amount depending on theboron content of the alloy. The amount precipitated in the form of Fe₂Bmay range up to 8% by volume. This second precipitation also takes placeaccording to a eutectic scheme, making it possible to obtain a fineuniform distribution, thereby ensuring good uniformity of the mechanicalproperties.

The precipitation of Fe₂B completes that of TiB₂, the maximum amount ofwhich is linked to the eutectic. The Fe₂B plays a role similar to thatof TiB₂. It increases the elastic modulus and reduces the density. It isthus possible for the mechanical properties to be finely adjusted byvarying the complement of Fe₂B precipitation relative to TiB₂precipitation. This is one means that can be used in particular toobtain an elastic modulus greater than 250 GPa in the steel and anincrease in the tensile strength of the product. When the steel containsan amount of Fe₂B equal to or greater than 4% by volume, the elasticmodulus increases by more than 5 GPa. The elongation at break is thenbetween 14% and 16% and the tensile strength reaches 590 MPa. When theamount of Fe₂B is greater than 7.5% by volume, the elastic modulus isincreased by more than 10 GPa, but the elongation at break is then lessthan 9%.

According to the invention, the mean size of the TiB₂ or Fe₂B eutecticprecipitates is equal to or less than 15 microns so as to obtain greaterelongation at break values and good fatigue properties.

When the mean size of these eutectic precipitates is equal to or lessthan 10 microns, the elongation at break may be greater than 20%.

The inventors have demonstrated that, when more than 80% by number ofthe TiB₂ eutectic precipitates are of single-crystal character, thematrix/precipitate damage when mechanically stressed is reduced and therisk of forming defects is less because of the greater plasticity of theprecipitate and its high level of cohesion with the matrix. Inparticular, it has been shown that larger TiB₂ precipitates formhexagonal crystals. Without wishing to be tied down by one particulartheory, it is believed that this crystallographic character increasesthe possibility of these precipitates deforming by twinning under theeffect of a mechanical stress.

This particular single-crystal character, due to the precipitation ofTiB₂ in a eutectic form, is not encountered to such a degree in theprocesses of the prior art, which are based on extrinsic additions ofparticles.

Apart from the favorable effect of a dispersion of endogenous particleson the tensile properties, the inventors have demonstrated that limitingthe grain size is a very effective means for increasing the tensileproperties: when the mean grain size is equal to or less than 15microns, the tensile strength may exceed about 560 MPa. In addition,when the grain size is equal to or less than 3.5 microns, the cleavageresistance is particularly high: Charpy toughness tests with a thicknessof 3 mm at −60° C. show that the ductile area in the test specimensfractured is greater than 90%.

The process for manufacturing a plate according to the invention isimplemented as follows:

-   -   a steel with the composition according to the invention is        supplied; and    -   the steel is then cast into a semifinished product.

This casting may be carried out to form ingots or carried outcontinuously to form slabs with a thickness of around 200 mm. It is alsopossible to cast the steel in the form of thin slabs a few tens ofmillimeters in thickness or thin strips a few millimeters in thicknessbetween counter-rotating rolls. The latter method of implementation isparticularly advantageous for obtaining a fine eutectic precipitationand to prevent the formation of primary precipitates. By increasing thecooling rate during solidification, the fineness of the microstructureobtained is increased.

Of course, the casting may be carried out in a format allowing themanufacture of products having various geometries, in particular in theform of billets for manufacturing long products.

The fineness of the TiB₂ and Fe₂B precipitation increases the tensilestrength, the ductility, the toughness, the formability and themechanical behavior in the HAZ. The fineness of the precipitation isincreased thanks to a low casting temperature and a higher cooling rate.In particular, it has been discovered that a casting temperature limitedto 40° C. above the liquidus temperature leads to such finemicrostructures being obtained.

The casting conditions will also be chosen in such a way that thecooling rate during solidification is equal to or greater than 0.1° C./sso that the size of the TiB₂ and Fe₂B precipitates are particularlyfine.

The inventors have also demonstrated that the morphology of the TiB₂ andFe₂B eutectic precipitates plays a role in the damage during subsequentmechanical solidification. After observing the precipitates under anoptical microscope and magnifications ranging from 500×1500×approximately on a surface having a statistically representativepopulation, the maximum size L_(max) and the minimum size L_(min) ofeach precipitate are determined using image analysis software known perse, such as for example the image analysis software Scion®. The ratio ofmaximum size to minimum size L_(max)/L_(min) characterizes the aspectratio f of a given precipitate. The inventors have demonstrated thatprecipitates of large size (L_(max)>15 microns) and of elongate shape(f>5) reduce the uniform elongation and the work-hardening coefficientn.

According to the invention, after the semi-finished product has beencast, the reheat temperature and reheat time for the semi-finishedproduct before subsequent hot rolling are chosen so as to cause the mostdeleterious precipitates to spheroidize. In particular, the reheattemperature and reheat time are chosen in such a way that the density ofelongate (f>5) eutectic precipitates with a size L_(max)>15 microns isless than 400/mm².

The semi-finished product then undergoes hot rolling, possibly followedby coiling. Optionally, cold rolling and annealing are carried out inorder to obtain thinner plates. The hot-rolling, coiling, cold-rollingand annealing conditions are chosen in such a way that a steel platewith a mean grain size equal to or less than 15 microns, preferably lessthan 5 microns and very preferably less than 3.5 microns, is obtained. Afiner grain size is obtained by:

-   -   substantial work-hardening before the end of hot rolling and        before the (γ-α) allotropic transformation that occurs upon        cooling;    -   a low end-of-rolling temperature, preferably below 820° C.;    -   accelerated cooling after the (γ-α) transformation so as to        limit ferritic grain growth;    -   a coiling operation at a relatively low temperature; and    -   after possible cold rolling, the annealing temperature and        annealing time are limited for the purpose of obtaining complete        recrystallization, without temperature and time exceeding the        values necessary for this recrystallization.

An end-of-hot-rolling temperature below 820° C. proves in particular tobe an effective means for obtaining a fine grain size. One particulareffect of the TiB₂ and Fe₂B precipitates on the nucleation andrecrystallization of the microstructures has been demonstrated in thesteels according to the invention. Specifically, when the steelsaccording to the invention are deformed, the significant difference inmechanical behavior between the precipitates and the matrix leads togreater deformation around the precipitates. This intense localdeformation reduces the non-recrystallization temperature. A lowend-of-rolling temperature promotes ferritic nucleation around theprecipitates and limits grain growth.

Likewise, the higher deformation field around the precipitates promotesgrain nucleation during the restoration/recrystallization that followsthe cold rolling, resulting in grain refinement.

The steel plate obtained in this way thus exhibits very goodformability. Without wishing to be tied down by one particular theory,it is believed that the eutectic precipitates present within a verydeformable matrix play a role similar to that played by martensitic orbainitic phases within the ferrite in “dual-phase” steels. The steelsaccording to the invention have a (yield strength R_(e)/tensile strengthR_(m)) ratio favorable to a variety of forming operations.

Depending on the carbon content and that of the hardening elements, anddepending on the cooling rate below the temperature Ar1 (thistemperature denoting the start of transformation upon cooling fromaustenite), it is possible to obtain hot-rolled plates or cold-rolledand annealed plates comprising matrices with variousmicrostructures—these may be completely or partially ferritic, bainitic,martensitic or austenitic.

For example, a steel containing 0.04% C, 5.9% Ti and 2.3% B will have,after being cooled from 1200° C., a Vickers hardness ranging from 187 to327 for a cooling rate ranging from 5 to 150° C./s. The highest hardnesslevels correspond in this case to a completely bainitic matrix composedof carbide-free slightly disoriented laths.

If it is desired to produce a part by a forming operation, a blank iscut from the plate and this is deformed by means such as drawing orbending in a temperature range between 20 and 900° C. The hardeningphases TiB₂ and Fe₂B exhibit very good thermal stability up to 1100° C.

Because of the thermal stability of the particles dispersed in thematrix and the suitability to the various cold, warm or hot formingprocesses, parts of complex geometry with an increased elastic modulusmay be produced according to the invention. Furthermore, the increase inthe elastic modulus of the steels according to the invention reduces thespringback after the forming operations and thereby increases thedimensional precision on finished parts.

Advantageously, structural elements are also manufactured by welding thesteels according to the invention, having identical or differentcompositions or identical or different thicknesses, so as in the finalstage to obtain parts whose mechanical properties vary within them andare adapted locally to the subsequent stresses.

Apart from iron and inevitable impurities, the composition by weight ofthe steels that can be welded to the steels according to the inventioncomprise, for example: 0.001-0.25% C; 0.05-2% Mn; Si≤0.4%; Al≤0.1%;Ti<0.1%; Nb<0.1%; V<0.1%; Cr<3%; Mo<1%; Ni<1%; B<0.003%, the balance ofthe composition consisting of iron and inevitable impurities resultingfrom the smelting.

In the melted zone, owing to the high temperature reached, theprecipitates partially dissolve and then reprecipitate upon cooling. Theamount of precipitates in the melted zone is very comparable to that ofthe base metal. Within the HAZ of the welded joints, the eutecticprecipitates are not dissolved and may even serve to slow down the rateof austenitic grain growth and of possible nucleation sites during thesubsequent cooling phase.

During a welding operation carried out on the steels according to theinvention, the concentration of TiB₂ and Fe₂B precipitates is thereforeuniform, going from the base metal to the melted metal passing throughthe HAZ, thereby guaranteeing, in the case of welded joints, that theintended mechanical properties (modulus, density) will be continuousthrough said joints.

To give a nonlimiting example, the following results demonstrate theadvantageous features conferred by the invention.

Example 1

Steels with the composition given in Table 1 below, expressed inpercentages by weight, were produced.

Apart from steels I-1 and I-2 according to the invention, this tableindicates, for comparison; the composition of a reference steel R-1 thatcontains no endogenous TiB₂ or Fe₂B eutectic precipitates.

These steels were produced by casting semifinished products from theliquid state, the additions of titanium and boron taking place in thecase of steels I-1 and I-2 in the form of ferro-alloys. The castingtemperature was 1330° C., i.e. 40° C. above the liquidus temperature.

TABLE 1 Steel compositions (wt %) B − Steel C S P Al Mn Si Ti B (0.45 ×Ti) I-1 0.0334 0.0004 0.007 0.263 0.069 0.084 4.50 1.68 −0.34 I-2 0.040.0015 0.009 0.146 0.09 0.14 5.90 2.34 −0.31 R-1 0.0023 0.008 0.0110.031 0.129 0.038 0.054 (*) — (*) 0 I = According to the invention; R =Reference; (*) = not according to the invention.

The microstructure in the as-cast state, illustrated in FIGS. 1 and 2,relating to steels I-1 and I-2 respectively, shows a fine uniformdispersion of endogenous TiB₂ precipitates within a ferritic matrix, Theboron has precipitated in the form of a binary Fe—TiB₂ eutectic.

The volume amounts of precipitates were measured by means of an imageanalyzer and are 9% and 12.4% for steels I-1 and I-2 respectively. Theamount of TiB₂ in the form of primary precipitates is less than 2% byvolume and promotes good castability. The mean sizes of the TiB₂eutectic precipitates are 5 and 8 microns for steels I-1 and I-2respectively. Among the population of these precipitates, more than 80%by number have a single-crystal character.

After being reheated to 1150° C., the semifinished products were thenhot-rolled into forms of plates down to a thickness of 3.5 mm, theend-of-rolling temperature being 940° C. The hot rolling was followed bycoiling at 700° C.

Treatments were also carried out by reheating steel I-2 to 1230° C.before hot rolling, for times varying from 30 to 120 minutes. Themorphology of the precipitates was then observed. It has been shown thata treatment at 1230° C. for a time of 120 minutes or longer enables theprecipitates to be spheroidized in such a way that the density of large(L_(max)>15 microns) elongate (f>5) eutectic precipitates is less than400/mm².

The uniform elongation A_(u) and the work-hardening coefficient n arethen significantly increased, since they go from 11% and 0.125 (reheattime: 30 minutes) to 16% and 0.165 (reheat time: 120 minutes) thanks tothe precipitate spheroidization treatment. Moreover, in the case ofsteel I-2, a plate was hot-rolled with an end-of-rolling temperature of810° C.

These hot-rolled plates were then pickled using a process known per seand then cold-rolled down to a thickness of 1 mm. They then underwentrecrystallization annealing at 800° C., with a 1 minute soak, beforebeing air cooled.

SEM (Scanning Electron Microscopy) observation showed no-loss ofcohesion at the matrix/eutectic precipitate interface or no damage ofthe precipitates themselves after hot rolling or cold rolling.

After hot rolling, the mean grain size of steel I-1 was 12 microns,whereas it was 28 microns in the case of the reference steel.

In the case of steel I-2, a low end-of-rolling temperature (810° C.)resulted in a finer mean grain size (3.5 microns) after hot rolling.

After cold rolling and annealing, the structure of steels I-1 and I-2was recrystallized, as indicated in FIG. 3 relating to steel I-1. Themicrograph was taken using a scanning electron microscope in crystallinecontrast mode, thereby attesting to the completely recrystallizedcharacter of the structure. The precipitates are very predominantlyeutectic precipitates. Compared with the conventional steel R-1, theTiB₂ precipitates cause substantial refinement of the microstructure—themean grain size is 3.5 microns for steel I-1 according to the inventionwhereas it is 15 microns in the case of the reference steel R-1.

Pycnometry measurements indicate that the presence of the TiB₂ and Fe₂Bprecipitates is associated with a significant reduction in the relativedensity d since this goes from 7.80 (conventional steel R-1) to 7.33(steel I-2).

The elastic moduli of steels I-1 and I-2 measured in the rollingdirection were 230 GPa and 240 GPa respectively. The elastic modulus ofthe reference steel R-1 was 210 GPa. For sheets stressed in bending, theperformance index of which varied as E^(1/3)/d, the use of the steelsaccording to the invention enabled a weight reduction of more than 10%over the conventional steels to be obtained.

The measured tensile properties (conventional yield strength R_(e)measured at 0.2% strain, tensile strength R_(m), uniform elongationA_(u) and, elongation at break A_(t)) are given in Table 2 (hot-rolledplates) or Table 3 (cold-rolled and annealed plates) below.

TABLE 2 Tensile properties of hot-rolled plates (parallel to the rollingdirection) R_(e) R_(m) A_(u) A_(t) Steel (MPa) (MPa) (%) (%) I-1 300 55815 22 I-2 244 527 14 20

TABLE 3 Tensile properties of cold-rolling and annealed plates (parallelto the rolling direction) R_(e) R_(m) A_(u) A_(t) Steel (MPa) (MPa) (%)(%) I-1 311 565 16 21 R-1 200 300 42 48

The R_(e)/R_(m) ratio of the hot-rolled or cold-rolled plates accordingto the invention is close to 0.5, resulting in mechanical behaviorapproaching that of a dual-phase steel and good capability of subsequentforming.

Spot resistance welding tests were carried out on cold-rolled plates ofsteel I-1: in tension-shear tests, failure systematically occurs bypeeling. It is known that this is a preferred fracture mode as it isassociated with a high energy.

It has also been shown that, within the melted zones in welding,eutectic precipitates according to the invention are present, therebyhelping to make the mechanical properties in welded assemblies uniform.

Satisfactory properties were also obtained in laser welding and arcwelding.

Example 2

Table 4 below shows the compositions of three steels according to theinvention.

TABLE 4 Compositions of steels according to the invention (wt %) Steel CMn Al Si S P Ti B B − (0.45 Ti) I-3 0.0465 0.082 0.15 0.17 0.0014 0.0085.5 2.8 0.32 I-4 0.0121 0.086 0.113 1.12 0.002 0.004 5.37 2.86 0.44 I-50.0154 0.084 0.1 0.885 0.0019 0.004 5.5 3.16 0.68

The steels were produced by casting semifinished products, the additionsof titanium and boron taking place in the form of ferro-alloys. Thecasting temperature was 40° C. above the liquidus temperature. Comparedwith steels I-1 and I-2, steels I-3 to I-5 have an excess amount ofboron compared to TiB₂ stoichiometry, in such a way that TiB₂ and thenFe₂B eutectic coprecipitations take place. The volume amounts ofeutectic precipitates are given in Table 5.

TABLE 5 Contents of precipitates (vol %) of steels I-3, I-4 and I-5.TiB₂ Fe₂B Steel (vol %) (vol %) I-3 13 3.7 I-4 12.8 5.1 I-5 13 7.9

The eutectic precipitates had an average size of less than 10 microns.FIG. 4 illustrates, in the case of steel I-3, the coexistence of TiB₂and Fe₂B precipitates. The light gray Fe₂B precipitates and the darkerTiB₂ precipitates are dispersed within the ferritic matrix.

The semifinished products were hot-rolled under conditions identical tothose presented in Example 1. Here again, no damage to theprecipitate-matrix interface was observed. FIG. 5 illustrates themicrostructure of steel I-5. The properties of these hot-rolled steelsare given in Table 6.

TABLE 6 Tensile properties (parallel to the rolling direction) andrelative density of hot-rolled plates. E R_(e) R_(m) A_(u) A_(t) Steel(GPa) (MPa) (MPa) (%) (%) d I-3 245 279 511 10 14 7.32 I-4 250 284 59011 14 7.32 I-5 254 333 585 8 9 7.30

Compared with steels I-1 and I-2, a complementary eutectic precipitationof Fe₂B in an amount by volume ranging from 3 to 7.9% increases theelastic modulus by an amount ranging from 5 to 15 GPa.

The complementary precipitation of Fe₂B increases the tensile strength.When this precipitation takes place in excessive proportions, theuniform elongation may however be markedly less than 8%.

Example 3

Semifinished products made of steel of composition I-2 were cast at atemperature of 1330° C. By varying the intensity of the flow for coolingthe semifinished products and the thickness of the cast semifinishedproducts, two cooling rates were achieved, i.e. 0.8° C./s and 12° C./s.The microstructures given in FIGS. 6 and 7 illustrate that an increasedcooling rate very significantly refines the Fe—TiB₂ eutecticprecipitation.

Example 4

Plates of steel with the composition I-2, 2.5 mm in thickness, werewelded by CO₂ laser welding under the following conditions: Power: 5.5kW; welding speed: 3 m/min. Micrographs of the melted zone show thatFe—TiB₂ eutectic precipitation takes place in a very fine form uponcooling from the liquid state. The amount of precipitates in the meltedzone is close to that of the base metal. Depending on the local coolingconditions during solidification (local temperature gradient G,displacement rate R of the isotherms), the solidification takes place indendritic form or in cellular form. The dendritic morphology isencountered more readily at the joint with the HAZ, given the localsolidification conditions (high gradient G and low rate R).

The TiB₂ precipitates are therefore present in the various zones of thejoint (base metal, HAZ and melted zone). Thus, the increase in elasticmodulus and the reduction in density occur throughout the welded joint.

A plate of steel I-2 was also laser welded without any operatingdifficulty with a plate of drawable mild steel, the composition of whichcontained (in wt %): 0.003% C, 0.098% Mn, 0.005% Si, 0.059% Al, 0.051%Ti, 0.0003% B and inevitable impurities resulting from the smelting. Themelted zone also contained Fe—TiB₂ eutectic precipitates, of course in alower proportion than in the case of autogenous welding. Consequently,it is possible to manufacture metal structures whose stiffnessproperties vary locally and whose mechanical properties correspond morespecifically to the local processing or service behavior requirements.

Example 5

Cold-rolled and annealed plates of steel I-2 according to the invention,with a thickness of 1.5 mm, were joined by resistance spot welding underthe following conditions:

-   -   assembly force: 650 daN;    -   welding cycle: 3 (7 periods with the current I flowing+2 periods        with no current flowing).

The welding range, expressed in terms of the current I, was between 7and 8.5 kA. The two bounds for this range correspond, on the one hand,to obtaining a core diameter greater than 5.2 mm (lower current bound)and, on the other hand, the appearance of sparking during welding (upperbound). The steel according to the invention therefore shows goodweldability by resistance spot welding with a sufficiently wide, 1.5 kA,weldability range.

The invention thus allows the manufacture of structural parts orreinforcing elements with an improved level of performance, both fromthe standpoint of intrinsic lightening and increase in elastic modulus.The easy processing of the steel plates according to the invention bywelding makes it possible to incorporate them into more complexstructures, in particular by means of joints with parts made of steelsof different composition or different thickness.

The automotive field will most particularly benefit from these variousfeatures.

What is claimed is:
 1. A steel plate, the chemical composition of which being expressed by weight: 0.010%≤C≤0.20%; 0.06%≤Mn≤3%; Si≤1.5%; 0.005%≤Al≤1.5%; S≤0.030%; P≤0.040%; and titanium and boron in amounts such that: 4.6%≤Ti≤6%; and (0.45×Ti)−0.35%≤B≤(0.45×Ti)+0.70%; optionally one or more of the following elements: Ni≤1%: Mo≤1%: Cr≤3%: Nb≤0.1%: and V≤0.1%: a balance of the composition consisting of iron and inevitable impurities resulting from smelting.
 2. The steel plate as claimed in claim 1, wherein the titanium and boron contents are such that: −0.22<B−(0.45×Ti)<0.35.
 3. The steel plate as claimed in claim 1, wherein the titanium and boron contents are such that: −0.35<B−(0.45×Ti)<−0.22.
 4. The steel plate of claim 1, wherein the C content expressed by weight being less than or equal to 0.080%.
 5. The steel plate of claim 1, wherein the C content expressed by weight being less than or equal to 0.050%.
 6. The steel plate of claim 1, wherein the composition comprises Cr, the Cr content expressed by weight being less than or equal to 0.08%.
 7. The steel plate of claim 1, further comprising TiB₂ precipitates, a mean size of which is equal to or less than 15 microns.
 8. The steel plate of claim 7, further comprising Fe₂B eutectic precipitates, a mean size of which is equal to or less than 15 microns.
 9. The steel plate of claim 1, further comprising TiB₂ precipitates, a mean size of which is equal to or less than 10 microns.
 10. The steel plate of claim 9, further comprising Fe₂B eutectic precipitates, a mean size of which is equal to or less than 10 microns.
 11. The steel plate of claim 9, wherein more than 80% by number of said TiB₂ precipitates are of single-crystal character.
 12. The steel plate of claim 1, wherein a mean grain size of said steel is equal to or less than 15 microns.
 13. The steel plate of claim 1, wherein a mean grain size of said steel is equal to or less than 5 microns.
 14. The steel plate of claim 1, wherein a mean grain size of said steel is equal to or less than 3.5 microns.
 15. The steel plate of claim 1, wherein its elastic modulus measured in the rolling direction is, equal to or greater than 230 GPa.
 16. The steel plate of claim 1, wherein its elastic modulus measured in the rolling direction is at least 240 GPa.
 17. The steel plate of claim 1, wherein its elastic modulus measured in the rolling direction is at least 250 GPa.
 18. The steel plate of claim 1, wherein its tensile strength is equal to or greater than 500 MPa and its uniform elongation is equal to or greater than 8%.
 19. The steel plate of claim 1 wherein the composition comprises Ni, the Ni content expressed by weight, Ni≤1%.
 20. The steel plate of claim 1 wherein the composition comprises Mo, the Mo content expressed by weight, Mo≤1%.
 21. The steel plate of claim 1 wherein the composition comprises Cr, the Cr content expressed by weight, Cr≤3%.
 22. The steel plate of claim 1 wherein the composition comprises Nb, the Nb content expressed by weight, Nb≤0.1%.
 23. The steel plate of claim 1 wherein the composition comprises V, the V content expressed by weight, V≤0.1%.
 24. An object manufactured from a plurality of steel parts, of identical or different composition and of identical or different thickness, wherein at least one of said steel parts is a steel plate as claimed in claim 1, which is welded to at least one of the other said steel parts, the composition or compositions of the other said steel parts comprising, by weight: 0.001%≤C≤0.25%; 0.05%≤Mn≤2.0%; Si≤0.4%; Al≤0.1%; Ti<0.1%; Nb<0.1%; V<0.1%; Cr<3%; Mo<1%; Ni<1%; B<0.003%, a balance of the composition being iron and inevitable impurities resulting from smelting.
 25. A structural part or reinforcement element for an automobile comprising: the steel sheet according to claim
 1. 26. A manufacturing process in which a steel according to claim 1 is supplied and said steel is cast in the form of a semifinished product, the casting temperature not exceeding more than 40° C. above the liquidus temperature of said steel.
 27. The manufacturing process as claimed in claim 26, wherein said semifinished product is cast in the form of a thin slab or thin strip between counter-rotating rolls.
 28. The manufacturing process of claim 26, wherein the cooling rate during solidification of said casting is equal to or greater than 0.1° C./s.
 29. The manufacturing process of claim 26, wherein said semi-finished product is reheated before it is hot-rolled, the temperature and the duration of said reheat both being chosen in such a way that the density of the TiB₂ and optionally Fe₂B eutectic precipitates, with a maximum size lmax greater than 15 microns and an aspect ratio f>5, is less than 400/mm², and said semi-finished product is hot-rolled.
 30. The process of claim 26, wherein a hot-rolling operation is carried out on said semifinished product, optionally a cold-rolling operation and an annealing operation, the rolling and annealing conditions being adjusted in such a way that a steel plate with a mean grain size equal to or less than 15 microns is obtained.
 31. The process of claim 26, wherein a hot-rolling operation is carried out on said semifinished product, optionally a cold-rolling operation and an annealing operation, the rolling and annealing conditions being adjusted in such a way that a steel plate with a mean grain size equal to or less than 5 microns is obtained.
 32. The process of claim 26, wherein a hot-rolling operation is carried out on said semifinished product, optionally a cold-rolling operation and an annealing operation, the rolling and annealing conditions being adjusted in such a way that a steel plate with a mean grain size equal to or less than 3.5 microns is obtained.
 33. The process of claim 29, wherein said hot-rolling operation is carried out with an end-of-rolling temperature below 820° C.
 34. A structural part or reinforcement element for an automobile comprising: the semifinished product manufactured by the process according to claim
 26. 